feat: DB-based round backups, restore voting system, and multi-server isolation

[hznovich]

What this does

1. Database-backed round backups

Round backups are now stored in the database (SQLite or MySQL) instead of JSON files on disk. The matchzy_backups table stores all backup metadata including the Valve backup payload.

• New table matchzy_backups for both SQLite and MySQL schemas
• SaveRoundBackup() writes directly to DB
• RestoreRoundBackup() reads from DB with legacy filename fallback
• GetBackups() / GetBackupInfo() use DB queries

2. Restore voting system

Non-admin players can now initiate a vote to restore a match from backup — useful when the server crashes and players reconnect.

• .restore starts a vote (available to all players when match is not live)
• Players vote via .yes / .no in chat
• 75% threshold, 20-second timeout
• On pass: shows 5 most recent backups from DB, player picks with .restore <id>
• Admin-only .restore unchanged when match is live

3. Multi-server isolation via matchzy_server_id

New ConVar matchzy_server_id — set per-server in server.cfg:

matchzy_server_id "moscow-bo1"

• Backups are tagged with server_id in the DB
• All queries filter by server_id when set
• Empty value = no filtering (backward compatible)

Files changed

• DatabaseStats.cs — new table, BackupRecord class, 5 CRUD methods
• BackupManagement.cs — DB-backed backup/restore, voting system (StartRestoreVote, OnRestoreVoteYes/No, ShowBackupListForVote)
• MatchZy.cs — .yes/.no commands, .restore → voting flow
• ConfigConvars.cs — new matchzy_server_id FakeConVar
• lang/en.json + lang/ru.json — 13 localization strings for voting

Backward compatibility

• Empty matchzy_server_id = no filtering (existing servers work)
• RestoreRoundBackup falls back to old filename format if not found in DB
• Existing file-based backup code path preserved (can be re-enabled)

All changes tested for compilation compatibility with CounterStrikeSharp API.